Electric discharge machining ("EDM") is a process for precisely manufacturing parts or molds (dies) by causing many electric discharges to occur at the gap while moving the tool electrode relative to a conductive workpiece. A continuous controlled power pulse is supplied to the gap in order to effect an electric discharge in the machining space or "gap" formed between the tool electrode and the workpiece (which forms another electrode).
In general, power pulses are generated in a power supply unit, which is unavoidably positioned at some physical distance from the machining unit, the latter including parts to support the workpiece and mechanical parts which cause the tool electrode to move relative to the workpiece. The power supply unit houses, typically within a cabinet, electric parts for generating the power pulse, as well as a controller which controls the size, polarity, ON and OFF time of the power pulse energy, and the relative movement between the tool electrode and workpiece. The power supply unit is connected to the machining unit by an appropriate lead line.
An electrode, shaped in the mirror image of the desired shape to be fabricated from the workpiece and made from a conductive material such as copper or graphite, is used as the tool electrode. Alternatively, a moving wire electrode made of a material such as brass may be used in order to cut out a desired profile from the workpiece. Machining using a moving wire electrode is analogous to mechanical machining with a fret or bead saw.
During machining, the workpiece is housed in a work tank, and the gap is filled with a dielectric fluid such as kerosene or high relative-resistance deionized water. When, by means of the application of power pulses to the gap, the voltage across the gap reaches a certain value, the dielectric fluid becomes ionized and a discharge occurs. As a result, a current flows through the gap, and a very small portion of the workpiece material evaporates or melts, whereupon it is washed away from the gap by the flow of dielectric fluid. In this manner, very small crater-like holes are formed on the surface of the workpiece. Insulation across the gap is restored by having the cessation of the power pulse application.
In general, after first performing rough machining under "low-wear" machining conditions, finish machining is performed in order to improve the quality of the workpiece surface by applying small energy power pulses to the gap. A technique is known whereby a dielectric fluid into which powdered metal or semi-metal material is supplied to the gap during the finish machining. A mixture of silicon powder having an average particle size of 20-40 .mu.m and mixed in kerosene at a ratio of 20 g/l, is disclosed in Japanese Laid-Open Patent 2-83119. Japanese Laid-Open Patent 3-277421 discloses the use of silicone geranium, and aluminum powder having an average particle size of 5-10 .mu.M.
An object of the present invention is to provide an electric discharge machining method for machining a workpiece with good dimensional accuracy and forming a pinhole-free surface thereon.
Another object of the present invention is to provide an electric discharge machining method to form a hard, mirror-finish surface on the workpiece.
Yet another object of the present invention is to provide an electric discharge machining fluid to the workpiece which will contribute to the forming of a pinhole-free mirror-finish surface.
Other objects of the present invention will in part be discussed in the explanation below, and will in part become clear to practitioners of the art through the implementation of the invention.